Hi ToniS1 ,
The change is outlined in the post, and if you have questions, it would be great if you can ask there instead of on this thread which is related to secure invoicing.
In short though, these older versions have been unsupported for a long time and are now the technology that supports them is being decommissioned which will 'retire' the software.
In real terms, if you stay in your V16 and V13 software, your files will fail confirmation in the near future and become read only.
If you need to interact with your files, you will need to upgrade to the latest version.